High dielectric constant thin film structure, method for forming high dielectric constant thin film, and apparatus for forming high dielectric constant thin film

ABSTRACT

There is provided a (Ba, Sr) TiO 3  film of higher dielectric constant and less leakage current for serving as a dielectric thin film of a capacitor in a semiconductor memory. DPM (dipivaloylmethanato) compounds of Ba, Sr and Ti are dissolved in THF (tetrahydrofuran) to obtain Ba(DPM) 2  /THF, Sr(DPM) 2  /THF and TiO(DPM) 2  /THF solutions which are used as source material solutions. A (Ba, Sr) TiO 3  film is formed by a CVD method while increasing a relative percentage of a Ti source material flow rate to a sum of Ba source material flow rate and Sr source material flow rate. The film formation is carried out in multiple steps, and annealing is applied in each step after deposition of the film.

This is a divisional of application Ser. No. 08/720,751 filed Oct. 1,1996, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for forming film a filmforming apparatus, and a thin film structure for use in the ChemicalVapor Deposition (CVD) method used in the process of forming a thin filmwith a high dielectric constant, such as is required in a semiconductormemory.

2. Description of the Related Art

In recent years, rapid progress has been made in the integration ofsemiconductor memories or devices and, particularly, with respect todynamic random access memories (DRAMs). In DRAMs, for example, theprogress in integration has been so rapid that the number of bits hasquadrupled in only three years, even against the background of theincreasing needs of the highly integrated device (e.g., reduced powerconsumption and cost).

No matter how much integration may be improved, however, it remains truethat the capacitor forming a memory cell of a DRAM must have asufficient capacity. To produce such a capacitor, it is necessary tomake a thin film of capacitor material. There exists, however, a currentlimit as to the minimum thinness achievable with a capacitor materialfilm, such as SiO₂.

A sufficient capacity may also be secured, in the same manner asthinning the film, by increasing the dielectric constant of thecapacitor material through variation of the material itself. In view ofthe limit on thinness, and with an eye toward increasing the dielectricconstant of the capacitor material, research and development intoutilizing a material with a high dielectric constant in a memory devicehas been increasingly popular in this technical field.

In terms of the performance required of such a capacitor material, whatis most important is that a film be thin and have a high dielectricconstant, as mentioned above, and that the leakage current be small.That is, in employing any material with a high dielectric constant, thematerial must be made as thin as possible, and the leakage currenttherefrom must be minimized. It is generally desired, as a roughdevelopment target, that the film thickness be 0.6 nm or less, ascalculated in terms of SiO₂, and that the leak current density be 2×10⁻⁷A/cm² or less when IV is applied.

In forming a thin film on an electrode for a stepped DRAM capacitor, aCVD method suitable for deposition on an object of such a complicatedshape is most advantageous. A serious problem, however, has hithertoexisted in that there is no known material which has a stable anddesirable vaporization characteristic which can be used as sourcematerial for CVD. This is mainly because the vaporization characteristicfrom heating a dipivaloylmethanato (DPM) compound of β-diketone(popularly used as a CVD source material) is not always satisfactory.

In view of the foregoing circumstances, the inventors already haveproposed a CVD source material for which the vaporization characteristicis remarkably improved. This improvement is accomplished by dissolvingconventional solid source materials in an organic solvent called THF(Tetrahydrofuran: C₄ H₃ O) to obtain a solution. This method isdisclosed in Japanese Patent Application No. Hei 4-289780.

A desirable result is not, however, always achieved merely by preparinga thin film having a high dielectric constant through employing theforegoing source materials when a conventional CVD apparatus for liquidsource materials is used (e.g., an apparatus for forming SiO₂ film). Inresponse to this problem, the inventors have also proposed a CVDapparatus for liquid source materials which is capable of sufficientlyvaporizing and feeding the liquid source materials. This apparatus isdisclosed in Japanese Laid-Open Patent Publications (unexamined) Nos.Hei 6-310444 and Hei 7-94426.

The inventors have also proposed that coverage can be remarkablyimproved by changing Ti source material from the popularly employed TTIPTi(O-i-Pr)₄ ! to DPM titanyl bis (dipivaloylmethanato) TiO (DPM)₂, beingthe same as Ba or Sr source material. As well, the inventors havediscerned that a two-step deposition process can be very effective forobtaining a desirable surface contour, and a desirable electriccharacteristic, especially as compared with a single layer film. The twosteps include a step of crystallizing an initial film (buffer layer) byannealing in the initial stage of film formation, when a relativelyamorphous film is easily formed, and a step of depositing a second layerfilm (main layer). These advances are disclosed in the JapaneseLaid-Open Patent Publication (unexamined) No. Hei 7-268634.

A problem still remains. Even when a high dielectric constant thin filmis prepared by using the above-mentioned solution vaporizing CVDapparatus, a desirable level of stability (including electriccharacteristic stability) of the thin film is not always achieved. FIG.1 is a schematic view of the solution vaporizing CVD apparatus shown inthe Japanese Laid-Open Patent Publication (unexamined) No. Hei 7- 94426.In the drawing, a (Ba, Sr)TiO₃ film is deposited by CVD. Referencenumeral 21 designates a substrate, 23 designates a source material gasfeed pipe, 24 is an oxidizing agent supply pipe, 31 is a vaporizer, 32is a reaction chamber, 41 is a dilution gas pipe, 42 is a dilution gasamount regulator, 43 is a pressure pipe, 44 is a liquid source materialcontainer, 45 is a liquid source material feeder, 46 is a connectionpipe, 47 is a pulverizing nozzle, 48 is a vaporizer heater, 49 is avaporizing chamber, 50 is a raw material gas feed port, 51 is a feedpipe heater, 52 is a reaction chamber heating mechanism, and 53 is asubstrate heater.

In the solution vaporization CVD apparatus of FIG. 1, a dilution gas N₂flows through the dilution gas pipe 41 and connection pipe 46, while theflow rate thereof is regulated by the dilution gas amount regulator 42.Solution source material in the liquid source material container 44 isfed from the pressure pipe 43, through the pulverizing nozzle 47, andinto the flowing dilution gas, while being pressurized and controlled bythe liquid source material feeder 45. It is then sprayed in thevaporizing chamber 49 of the vaporizer 31. The source material gasvaporized in the vaporizer is fed from the source material gas feed port50, through the source material gas feed pipe 23, which is heated by thefeed pipe heater 51, and to the reaction chamber 32. After reaction withan oxidant in the reaction chamber, a (Ba, Sr) TiO₃ film is prepared onthe substrate, which is heated by the substrate heater 53.

In the actual apparatus, three liquid source material feed systems(components 43 to 45, as shown in FIG. 1) are respectively provided forBa, Sr and Ti, and the source materials are fed into one vaporizer. Inthe reaction chamber, the flow rate of the source materials and the filmformation time are controlled under the following conditions: atmosphereof O₂, pressure of 1 to 10 Torr, and temperature of 400° to 600° C. Thetemperature is relatively low because a low temperature provides bettercoverage. Thereby, a film is formed according to an established targetof obtaining a (Ba, Sr) TiO₃ film having a component ratio of(Ba+Sr)/Ti=1.0, and a film thickness of 300 Å at a deposition rate of 30Å/min.

When forming a (Ba, Sr) TiO₃ film (BST film, hereafter) by a CVD methodemploying liquid source materials composed of any DPM organo-metalliccompound dissolved in an organic solvent, even if a constant flow rateof the respective source materials (i.e., Ba, Sr, and Ti) isestablished, there is still the problem of heterogeneous, or uneven,distribution of the Ba, Sr and Ti components in the direction of filmthickness. Also, there can be the mixture, or contamination, of the filmwith carbon, which eventually results in problems such as an unstableelectric characteristic, a lowering of the dielectric constant, anincrease in the leakage current, a decrease in the film's voltageresistance, etc.

The following section focuses on this problem of the heterogeneouscomponent distribution of Ba, Sr, and Ti.

The conventional solution vaporizing CVD apparatus is constructed suchthat the DPM compounds containing Ba, Sr or Ti are each stored indifferent containers, and the supply thereof to the reactor isindependently decided. During the deposition of a composite oxide filmof those metallic elements on a substrate, the substrate is preciselykept at a certain temperature, within a range of 5° C. or less, byheating a substrate support section with a resistance heater. However,when depositing a BST composite oxide film with this type of filmforming apparatus, and with the substrate temperature kept at 420° C.,and with the component distribution of the metallic elements, in thefilm thickness direction, being measured by Auger Electron Spectroscopy(AES), a heterogeneous distribution (as shown in FIGS. 2 and 3) occurs.In each of FIGS. 2 and 3, the abscissa indicates the sputtering time forremoving the film, and the ordinate indicates the height of a peak valuerepresenting the existence of each element.

FIG. 2 is a component distribution diagram obtained by AES, in thedirection of film thickness, of the Ba, Sr, and Ti of a BST film formedvia CVD (CVD-BST film, hereafter) for two minutes on a Ru electrode witha constant solution flow rate. The solution was obtained by dissolvingsolid Ba, Sr, and Ti source materials Ba(DPM)₂. Sr(DPM)₂ and TiO(DPM)₂respectively in THF (Tetrahydrofuran: C₄ H₈ O), an organic solvent. Inthis film formation process, the deposition rate was about 30 Å/min, andthe film thickness was about 60 Å. FIG. 2 shows a trend that the nearerthe Ru electrode, the Ti in the film increases, while the Ba and the Srdecrease. This trend, or phenomenon, takes place even though the flowrates of the respective source materials Ba, Sr, and Ti are constant.FIG. 3 shows another component distribution diagram, in the filmthickness direction, of a CVD-BST film formed under the same conditions,but on a Pt electrode.

In the foregoing process for forming CVD-BST film, a problem exists inthat the amount of the CVD-BST film formed is different, depending onthe substrate material (hereinafter referred to as a dependency on thesubstrate). Even with a substrate made of plural materials and having apatterned surface, there is a the similar problem in that the amount offilm formed is different from the amount formed on a single materialsubstrate.

Another problem exists in that the surface temperature of the substrate21 (see FIG. 1) changes due to the deterioration, etc. of a susceptorwhich holds the heater 53 and the substrate 21, making it impossible toform film under constant temperature conditions for an extended periodof time.

Yet another problem exists in that it is impossible to perform a filmformation constantly due to clogging, or the like, which occurs duringthe process of mixing any source material solution with a dilution gas,and supplying it to a vaporizer.

A further problem exists in that the leakage current is large in a thinfilm structure that is comprised of a conventional thin film having ahigh dielectric constant, and the electrodes that hold the thin filmtherebetween.

FIG. 4 is a sectional view showing a stacked capacitor for a DRAM whichhas a thin film of a high dielectric constant as the dielectricmaterial. This arrangement is shown in Pierre C. Fazan. "Trends in thedevelopment of ULSI DRAM capacitors", Integrated Ferroelectrics 1994,Vol. 4, pp. 247-256. In FIG. 4, reference numeral 1 designates a siliconsubstrate; 33 is a cell plate made of platinum, for example; 34 is afilm of high dielectric constant; 35 is a storage node made of platinum,for example; 37 is an interlayer insulating film made of silicondioxide, for example; and 38 is a plug of polysilicon, for example, forelectrically connecting the storage node to, for example, a transistor.

Applying the high dielectric constant film CVD to the practicalformation of the capacitor shown in FIG. 4 involves the following steps:burying a polysilicon film by CVD in holes, formed by lithography andetching, on the interlayer insulating film 37; forming the plug 38 byremoving a portion of the interlayer insulating film 37 deposited on thesurface through full surface etching or a chemical mechanical polishingmethod (CMP method); forming the storage node 35, which is 200 nm inheight, by a lithography and etching process of platinum film depositedby sputtering on the plug 38; coating the entire surface of the storagenode 35 with the BST film 34, with a thickness of 30 nm, by a CVDmethod; and forming the cell plate 33 by depositing a platinum filmthrough sputtering. In the capacitor thus formed, a voltage appliedbetween the cell plate 33 and the storage node 35 causes a largeelectric charge to be stored on the upper and side faces of the storagenode 35, due to the high dielectric constant (of about 200) of the BSTfilm 34.

In the above case, in which CVD-BST film was applied to a conventionalcapacitor structure, when a voltage is applied between the cell plate 33and the storage node 35, a large field strength is generated in the BSTfilm near the storage node. This field strength is several times greaterthan the field strength in the remaining part of the BST film, and isdue to field concentration. The field concentration results since thethickness of the BST film 34, amounting to 30 nm, is large when comparedwith the radius of curvature of the shoulder portion 36 of the storagenode 35. The radius of curvature of the shoulder portion normally isonly about 5 nm. Consequently, when a negative voltage is applied to thestorage node, the leakage current due to the Schottky emission currentfrom the storage node 35 increases considerably, making it impossible tohold the electric charge.

A similar field concentration appears also at a lower end 39 of the cellplate 33. Therefore, just as in the case of a negative voltage beingapplied to the cell plate 33, the leakage current due to Schottkyemission current from the cell plate increases considerably, making itimpossible to hold the electric charge.

Further, since the BST film 34 is 30 nm in thickness, which is largewhen compared with the storage node 35 being 200 nm in height, the cellplate 33 is not able to coat the entire side face of the storage node35. As a result, the entire side face of the storage node 35 is noteffectively utilized as electrode of the capacitor. A still furtherproblem exists in that the potentials of adjacent storage nodesinterfere with, or negatively affect, each other through the BST film inthe cutout portion.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to form a highdielectric constant thin film having a homogeneous componentdistribution of Ba, Sr and Ti, in film thickness direction.

Another object of the invention is to form a high dielectric constantthin film which is high in dielectric constant and low in leakagecurrent.

A further object of the invention is enhance the thin film productionprocess by preventing clogging by a source material solution which ismixed with a dilution gas and supplied.

A yet further object of the invention is stably to form a highdielectric constant thin film, irrespective of variations in thesubstrate surface temperature.

A still further object of the invention is to be free from thedependency on substrate in the formation of a high dielectric constantthin film on the basis of surface material of substrate and coatingrate.

In accordance with the invention, a method will be described for forminga high dielectric constant thin film. In the method, a BST film isformed by CVD, using a solution obtained by dissolving a Ba DPM compound(serving as Ba source material), a Sr DPM compound (serving as Sr sourcematerial), and a Ti DPM compound (serving as Ti source material)respectively in an organic solvent. The method of the invention ischaracterized in that the ratio of the Ti source material flow rate tothe sum of the Ba and Sr source material flow rate is increased duringformation of the film.

Using this method, it is possible to form a thin film having a highdielectric constant, and having a homogeneous component distribution ofBa, Sr, and Ti in the film thickness direction.

When performing a multi-stage (multi-step) film deposition and annealingthe same after the deposition in each stage (step), a high dielectricconstant thin film of less leakage current may be obtained.

In another method for forming a high dielectric constant thin film, inaccordance with the invention, a thin film is formed by CVD using asolution obtained by dissolving a Ba DPM compound serving as Ba sourcematerial, a Sr DPM compound serving as Sr source material, and a Ti DPMcompound serving as Ti source material, respectively, in an organicsolvent. This method is characterized by the repeating the alternateformation of SrTiO₃ film and BaTiO₃ film. Using this method, it ispossible to obtain a high dielectric constant thin film with a reducedleakage current.

In another method for forming a high dielectric constant thin film, inaccordance with the invention, a (Ba, Sr) TiO₃ film is formed by CVDsupplying a source material solution obtained by dissolving a Ba DPMcompound serving as Ba source material, a Sr DPM compound serving as Srsource material, and a Ti DPM compound serving as Ti source material,respectively, in tetrahydrofuran together with a nitrogen gas. Thismethod is characterized in that, after adding tetrahydrofuran to thenitrogen gas flow, the Ba source material, Sr source material and Tisource material are added thereto to obtain a gas-liquid mixture, andthis gas-liquid mixture is then supplied. Using this method, the sourcematerial solution is prevented from clogging.

In yet another method for forming a high dielectric constant thin film,in accordance with the invention, a high dielectric constant thin filmof metal oxide is formed on a substrate by CVD employing two or moreorgano-metallic compounds as source materials. This method ischaracterized in that the deposition of the metal oxide by decompositionof at least one of the organo-metallic compounds depends on the surfacetemperature of the substrate, and the composition ratio of metallicelements in the deposited film is varied in the film thickness directionby varying a substrate surface temperature during the deposition of themetal oxide thin film. With this method, it is possible to obtain acomposition ratio of steep inclination among the metallic elements. Sucha ratio is difficult to obtain merely by adjusting the flow rate of thesource materials. It is also possible to reduce the leakage current.

In another method for forming a high dielectric constant thin film, inaccordance with the invention, a barium strontium titanate (Ba, Sr) TiO₃thin film is formed by CVD on a substrate coated with differentmaterials. This method includes the following steps:

(1) a first step of varying, as parameters, a) the kind and the coatingrate of a material with which substrate surface is coated, b) thethickness of the coating material, c) the thickness of the substrate, d)the temperature setting of a heater for heating the substrate, and e)the flow rate of Ti source material, while keeping the respective flowrates of Ba source material and Sr source material at a certain value,depositing a thin film on the substrate, and measuring a compositionratio of the metallic elements;

(2) a second step of measuring values of parameters (1) a) to c),deciding a value of d), establishing a target value of the compositionratio of the metallic elements, and obtaining flow rate of Ti sourcematerial required to achieve the target value, prior to actual formationof thin film; and.

(3) a third step of forming a thin film in accordance with the flow rateof Ti source material obtained in the second step and the flow rates ofBa source material and Sr source material established in the first step.

In this method, it is possible to be free from the dependency onsubstrate in the formation of a (Ba, Sr) TiO₃ thin film on the basis ofsubstrate surface material and coating rate.

A high dielectric constant thin film manufacturing apparatus inaccordance with the invention includes: a reaction chamber in which athin film is formed by CVD on a substrate; a source material gas feedpipe for feeding a source material gas to the reaction chamber; anoxygen gas feed pipe for feeding an oxygen gas to the reaction chamber;and an infrared sensor disposed in the oxygen gas feed pipe fordetecting a surface temperature of the substrate. With this apparatus,even if the substrate surface temperature varies, a high dielectricconstant thin film may be stably formed. Further, the DPM sourcematerials may be stably monitored without solidifying on the top end ofthe sensor.

A high dielectric constant thin film structure, in accordance with theinvention, comprises: an interlayer insulating film disposed on asemiconductor substrate and having a concavity on a part of a surface; aconductive plug disposed in a hole provided through the interlayerinsulating film; a lower electrode disposed on the plug and on theinterlayer insulating film; a high dielectric constant thin film formedon the lower electrode and on the concavity of the interlayer insulatingfilm; and an upper electrode formed on the high dielectric constant thinfilm. With this structure, it is possible to increase the electriccapacity of the high dielectric constant thin film, to prevent fieldconcentration, and to restrain the leakage current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a conventional method and apparatusfor forming a high dielectric constant thin film.

FIG. 2 is a component distribution diagram in depth direction (i.e., thefilm thickness direction) of a BST film formed on a Ru electrode by aconventional method for forming the high dielectric constant thin film.

FIG. 3 is a component distribution diagram in depth direction of a BSTfilm formed on a Pt electrode by the conventional method for forming thehigh dielectric constant thin film.

FIG. 4 is a sectional view showing a structure of the conventional highdielectric constant thin film.

FIG. 5 is a diagram showing a X-ray fluorescent spectroscopy strength ofBa, Sr and Ti of a BST film formed by supplying certain Ba, Sr and Tisolution source materials and varying a substrate temperature.

FIG. 6 is a diagram showing a composition, in the film thicknessdirection, of a BST film formed as an example in accordance with theinvention.

FIG. 7 is a diagram showing a flow rate of the source material used inan example in accordance with the invention.

FIG. 8 is a schematic view of a circuit for measuring the electriccharacteristic of a BST film.

FIG. 9 is a sectionally schematic view of the BST film formed in anexample in accordance with the invention.

FIG. 10 is a diagram showing a flow rate of the source martial used inan example in accordance with the invention.

FIG. 11 is a sectionally schematic view of the BST film formed in anexample in accordance with the invention.

FIG. 12 is a diagram showing a flow rate of the source martial used inseveral examples in accordance with the invention.

FIG. 13 is a sectionally schematic view of the BST film formed in anexample in accordance with the invention.

FIG. 14 is a diagram showing a relation between annealing temperatureand electric characteristic of the BST film formed in an example inaccordance with the invention.

FIG. 15 is a diagram showing temperature rise degassing analysis of theBST film formed in an example in accordance with the invention.

FIG. 16 is a diagram showing a relation between annealing temperatureand CO₂ peak strength of the BST film formed in an example in accordancewith the invention.

FIG. 17 is a sectionally schematic view of the BST film formed in anexample in accordance with the invention.

FIG. 18 is a sectionally schematic view of the BT film and ST filmformed in an example in accordance with the invention.

FIG. 19 is a diagram showing a flow rate of the source material used inthe formation of the film shown in FIG. 14.

FIG. 20 is a diagram showing a composition in film thickness directionof a BST film formed in an example in accordance with the invention.

FIG. 21 is a diagram showing a relation between a target ratio(Ba+Sr)/Ti of the upper substrate film and electric characteristic in anexample in accordance with the invention.

FIG. 22 is a view showing how to measure wafer temperature in an examplein accordance with the invention.

FIG. 23 is a diagram showing a relation between heater set temperatureand substrate surface temperature in an example in accordance with theinvention.

FIG. 24 is a diagram showing a relation between coating rate ofsubstrate surface material and substrate surface temperature in anexample in accordance with the invention.

FIG. 25 is a diagram showing a X-ray fluorescence spectroscopy strengthof Ba, Sr and Ti of a BST film formed by varying wafer surfacetemperature.

FIG. 26 is a diagram showing a X-ray fluorescence spectroscopy strengthof Ba, Sr and Ti of a BST film formed by changing the wafer surfacetemperature and the Ti source material flow rate.

FIG. 27 is a schematic view showing an example of a thin filmmanufacturing apparatus used in an example in accordance with theinvention.

FIG. 28 is a schematic view showing an example of a thin filmmanufacturing apparatus used in an example in accordance with theinvention.

FIG. 29 is a sectional view showing a structure of the high dielectricconstant thin film prepared in an example in accordance with theinvention.

FIGS. 30 (a), (b), (c), (d) and (e) are schematic views showing amanufacturing process of the structure of the high dielectric constantthin film shown in FIGS. 25 and 26.

FIG. 31 is a diagram showing three-dimensional electric field strengthsimulation result in the high dielectric constant thin film formed in anexample in accordance with the invention.

FIG. 32 is a diagram showing electric characteristic of the highdielectric constant thin film formed in an example in accordance withthe invention.

FIG. 33 is a sectional view showing a structure of the high dielectricconstant thin film formed in an example in accordance with theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

To clear up the cause of the heterogeneous element distribution of Ba,Sr, and Ti in a BST film, a manner of variation in thermal decompositionvelocity of DPM compound containing Ba, Sr, or Ti, with respect totemperature, was clarified through experimentation. FIG. 5 shows thedependency of deposition rate on substrate temperature during depositionof a thin film composed of a single oxide of each metallic elementemploying a DPM compound containing Ba, Sr or Ti. It is understood fromFIG. 5 that the deposition rate of the DPM compound containing Ti islargely dependent on substrate temperature as compared with theremaining DPM compounds containing Ba or Sr, and the deposition rate ofthe DPM compound containing Ti varies importantly due to a smallfluctuation of the substrate temperature in a range of several °C.Experimentation has shown that the deposition rate at the time ofsimultaneously introducing DPM compounds, respectively containing Ba,Sr, or Ti, in a reaction container and of depositing a film composed ofa composite oxide containing the respective metallic elements, is a sumof each deposition rate of a simple oxide of Ba, Sr, or Ti.

From this experimental result, it is understood that thermaldecomposition characteristic of one DPM compound containing Ba, Sr, orTi remains substantially unchanged, irrespective of existence of theother two. This means that an improvement in the uniformity, orhomogeneity, in film composition of a composite oxide film may beachieved simply by stabilizing each deposition rate of the DPM compoundcontaining Ba, Sr, or Ti.

FIG. 6 is a component distribution diagram, in the film thicknessdirection, of a buffer layer of CVD-BST film of 60 Å in film thickness.The diagram shows an example of the high dielectric constant thin filmwhich can be formed by applying a method in accordance with theinvention; the component distribution diagram was obtained by AES. Inthis method, as shown in FIG. 7, the flow rate of source material isvaried with the passage of time during the film formation. Morespecifically, flow rate of the Ti raw material solution is increasedwith the passage of time, while that of the Ba and Sr raw materialsolutions is reduced, forming a film of homogeneous composition in thefilm thickness direction.

In particular, a lower electrode is formed by sputtering Ru. Then, afirst BST film of homogeneous composition, in the film thicknessdirection, is formed on the lower electrode by varying the flow rate ofthe source material solutions as shown in FIG. 7. This first BST film isannealed under a nitrogen atmosphere. A second BST film is formed on thefirst BST film by CVD. The second BST film is annealed under thenitrogen atmosphere, and an upper electrode is formed by sputtering Ruon the annealed second BST film. The first BST film will hereinafter bereferred to as a buffer layer, and the second BST film as a main layer.

Composition of the buffer layer is even, or homogeneous, in the filmthickness direction, as shown in FIG. 6. It is also preferable to formthe lower and upper electrodes by sputtering Pt in place of Ru.

The flow rate of the Ba, Sr, and Ti source material solutions isconstant at the time of forming the main layer on the conditions shownin FIG. 1, and a total film thickness of the buffer layer and the mainlayer is about 240 Å. The annealing conditions of the buffer layer are640° C., N₂ atmosphere and 10 sec, and those of the main layer are 625°C., N, atmosphere and 10 sec. As for the annealing condition of thebuffer layer, 630° C. to 650° C. is preferable in view of sufficientcrystallization. As for the main layer, it is certain that annealing isnecessary for crystallization, but leakage current is increased if theannealing temperature is excessively high and, therefore, 610° C. to640° C. is preferable for annealing the main layer.

The conditions for forming the buffer layer, except for the respectiveflow rates of Ba, Sr, and Ti source material solutions, are as shown inTable 1.

                  TABLE 1    ______________________________________    Source material flow                 Ba(DPM).sub.2 /THF (0.1 mol/L)                                  0.04 cm.sup.3 /min    rate         Sr(DPM).sub.2 /THF (0.1 mol/L)                                  0.03 cm.sup.3 /min                 TiO(DPM).sub.2 /THF (0.1 mol/L)                                  0.5 cm.sup.3 /min    Carrier gas flow rate                 Carrier N.sub.2 flow rate                                  200 sccm    Vaporization Vaporizer temperature                                  250° C.    temperature    Vapor pressure                 Vaporizer pressure                                  20 Torr    Oxidizing agent flow                 O.sub.2 flow rate                                  1 slm    rate    Reaction capacity                 Reactor pressure 1.5 Torr    pressure    Substrate temperature                 Substrate temperature                                  420° C.    Substrate material                 Ru(2000Å)/SiO.sub.2                                  (5000Å)/Si                 or Pt(700Å)/SiO.sub.2                                  (5000Å)/Si    ______________________________________

A target flow rate of the Ba, Sr, and Ti source material solutions inTable 1 is a composition ratio (Ba+Sr)/Ti=0.8. In Table 1, anothercomposition ratio (Ba+Sr)/Ti=1.0 may be targeted either by reducing theflow rate of Ti source material solution by 20% or by increasing theflow rate of Ba and Sr source material solutions by 25%.

A further composition ratio (Ba+Sr)/Ti=1.2 may be targeted either byreducing the flow rate of Ti source material solution by 33% or byincreasing the flow rate of Ba and Sr source material solutions by 50%.In this manner, by either increasing or reducing the flow rate of Ba,Sr, and Ti source material solutions, any composition ratio of(Ba+Sr)/Ti may be freely established. Ba(DPM)₂ /THF, Sr(DPM)₂ /THF, andTiO(DPM)₂ /THF are solutions obtained by dissolving bis(dipivaloylmethanato) barium Ba(DPM)₂ !, bis (dipivaloylmethanato)strontium Sr(DPM)₂ ! and titanyl bis (dipivaloylmethanato) TiO(DPM)₂ !respectively in tetrahydrofuran.

The electric characteristic of the BST film formed as described above isthen measured. The upper electrode, Pt or Ru of 1 mm in diameter, isfurther formed by sputtering on the BST film formed over the lowerelectrode, also Pt or Ru, and the electric characteristics such asleakage current, film thickness calculated in terms of oxide film, etc.,are measured by means of an electrical circuit as shown in FIG. 8. Inthis drawing, reference numeral 1 designates a Si substrate, 2designates a SiO₂ film, 59 is a cell plate (upper electrode), 58 is astorage node (lower electrode), 54 is an impedance analyzer, and 55 is aCVD-BST film.

Table 2 shows the result of measuring the leakage current in a structureproduced as described above in comparison with the conventional art. Itis understood from Table 2 that the film, of which the buffer layer hasa homogeneous component distribution, in the film thickness direction,produced in accordance with the invention, is superior to a comparativeexample 1, which has a buffer layer with a conventional componentdistribution in the film thickness direction. The structure producedaccording to the invention has a superior electric characteristic.

                  TABLE 2    ______________________________________           Film thickness t.sub.eq calculated                            Leakage current J.sub.L           in terms of oxide film                            (at +1.1 V)    ______________________________________    Comparative             0.56 nm            8.1 × 10.sup.-8 A/cm.sup.2    example 1    Example 1             0.50 nm            2.1 × 10.sup.-8 A/cm.sup.2    ______________________________________

EXAMPLE 2

FIG. 9 is a sectional view of a film showing another example of the highdielectric constant thin film forming method in accordance with theinvention. In the drawing, reference numeral 1 designates a Sisubstrate, 2 designates a SiO₂ film, 3 is a lower Ru electrode, 4 is aninitial BST film deposited for two minutes at a target composition ratioof (Ba+Sr)/Ti=1.2 to 1.4, and 5 is an upper BST film deposited for sixminutes at a target composition ratio of (Ba+Sr)/Ti=0.6 to 0.8.

A two-step film formation involves forming a main layer on a bufferlayer obtained by crystallizing a BST film by annealing in the initialstage of film formation, when a relatively amorphous film is easilyformed. It is effective in such a process to vary the ratio of totalflow rate of Ba and Sr solution source materials to that of Ti solutionsource material depending upon whether the buffer layer or the mainlayer is being formed, as shown in FIG. 10. Thereby, it is possible toobtain a film in which the sum of the composition ratio of Ba and Sr toTi is relatively homogeneous in film thickness. In FIG. 10, the flowrate of the source materials for forming the buffer layer is shown forfirst two minutes, and the flow rate of the source materials for formingthe main layer is shown for six minutes thereafter. The film formationconditions are same as those shown in FIG. 1, except for the flow rateof the source materials. Table 3 shows the respective electriccharacteristics of two CVD-BST films. The first was obtained as exampleaccording to the invention, and the second was produced according to theconventional method. In the comparative example, both of the bufferlayer and main layer were formed at a target ratio of (Ba+Sr)/Ti=0.7 to0.8. It is understood from Table 3 that the invention is advantageous.The annealing condition of the buffer layer and the main layer were thesame as those of example 1.

                  TABLE 3    ______________________________________           Film thickness t.sub.eq calculated                            Leakage current J.sub.L           in terms of oxide film                            (at +1.1 V)    ______________________________________    Comparative             0.56 nm            8.1 × 10.sup.-8 A/cm.sup.2    example 1    Example 2             0.52 nm            2.5 × 10.sup.-8 A/cm.sup.2    ______________________________________

EXAMPLE 3

FIG. 11 is a sectional view of a BST film formed according to a furtherexample of the high dielectric constant thin film forming method, inaccordance with the invention. In the drawing, reference numeral 6designates a Pt electrode, 7 is an initial BST film deposited at atarget composition ratio of (Ba+Sr)/Ti=0.9 to 1.1, and 5 is an upper BSTfilm deposited at a target composition ratio of (Ba+Sr)/Ti=0.7 to 0.8.In forming a film in two steps, it is effective to vary the ratio of thetotal flow rate of the Ba and Sr solution source materials to that ofthe Ti solution source material depending upon whether the buffer layeror the main layer is formed, as shown in FIG. 12. Thereby it is possibleto obtain a film in which the sum of the composition ratio of Ba and Srto Ti is relatively homogeneous in film thickness direction. Theannealing condition of the buffer layer and the main layer were the sameas those of example 1.

Referring to FIG. 3, in which is shown an AES component distributiondiagram of the film thickness of the buffer layer (formed for twominutes on Pt according to the conventional method), the position of thelargest peak is located near the Pt electrode side. Perhaps this isbecause Ti enters inside Pt. Accordingly, it is preferable that, in thecomposition of the buffer layer, Ti is applied to Pt in a larger amountas compared with that of example 1, in which a Ru electrode was used. Inother words, the composition ratio (Ba+Sr)/Ti should be smaller. Thefilm formation conditions are same as those shown in FIG. 1, except forthe flow rate of the source materials. Table 4 shows the electriccharacteristics of two CVD-BST films, the first having been obtainedaccording to the invention, and the second having been producedaccording to the conventional method, and in the comparative example,both of the buffer layer and the main layer were formed at a target of(Ba+Sr)/Ti=0.7 to 0.8. It is understood from Table 4 that the inventionis advantageous.

                  TABLE 4    ______________________________________           Film thickness t.sub.eq calculated                            Leakage current J.sub.L           in terms of oxide film                            (at +1.1 V)    ______________________________________    Comparative             0.60 nm            1.7 × 10.sup.-7 A/cm.sup.2    example 2    Example 3             0.56 nm            4.5 × 10.sup.-8 A/cm.sup.2    ______________________________________

EXAMPLE 4

FIG. 13 is a sectional view of a film deposited on a Pt electrodeaccording to a further example of the high dielectric constant thin filmforming method in accordance with the invention. In the drawing,reference numeral 8 designates an upper BST film annealed once at atemperature of 610° to 640° C. Usually, the annealing conditions of themain layer are: 625° C. (in the range of 610° to 640° C.), N₂atmosphere, and 10 sec. The usual annealing conditions of the bufferlayer are: 640° C., N₂ atmosphere, and 10 sec. The flow rate of thesolution source materials is same as that shown in FIG. 12, and theother conditions of film formation are same as those shown in Table 1.

FIG. 14 shows the relation between the annealing temperature andelectric characteristic of the main layer, and it is understood fromFIG. 14 that the electric characteristic at an annealing temperature of625° C. is most preferable, which shows an advantage of the invention.FIG. 15 shows a result of mass spectrometry of temperature rise anddegassing of CVD-BST film without annealing the main layer, and it is tobe understood from FIG. 15 that CO₂ (m/e=44) and H₂ O (m/e=18) aredetected as degassing at about 150° C. FIG. 16 shows a relation betweenthe peak intensity of CO₂ and the annealing temperature of the mainlayer. It is to be understood from FIG. 16 that carbon and similarcontaminants. Mixed into the BST film are removed by annealing, and anannealing temperature of about 625° C. is required in order sufficientlyto remove them. It is presumed that the carbon compound in the film isdecomposed by annealing, and is discharged in the form of CO₂, wherebythe leakage current can be further reduced.

EXAMPLE 5

FIG. 17 is a sectional view of a film formed up to a third layer (thirdstage) by multilayer (multistage) film formation on a Pt electrode inaccordance with the invention. In FIG. 17, reference numeral 9designates a buffer layer to which annealing has been applied threetime, 10 designates a second film to which annealing has been appliedtwice, and 11 is a third film to which annealing has been once. Theannealing conditions of the buffer layer are 640° C., N₂ atmosphere, and10 sec. The annealing conditions of the second and third films are 625°C., N₂ atmosphere, and 10 sec. The flow rate of source materialsolutions is same as that shown in FIG. 12, and the other film formationconditions are same as those shown in Table 1. It takes two minutes todeposit the buffer layer, three minutes to deposit the second film andthree minutes to deposit third film. By performing annealing threetimes, the carbon amount remaining at 625° C. is further reduced, asshown in FIG. 16. Table 5 shows the electric characteristic of thisexample 5 in comparison with the two step film formation according toexample 3, and it is to be understood that the leakage current issmaller. This example clearly is more effective and advantageous. It ispresumed that carbon is reduced by repeating the annealing, wherebyleakage current is reduced.

                  TABLE 5    ______________________________________           Film thickness t.sub.eq calculated                            Leakage current J.sub.L           in terms of oxide film                            (at +1.1 V)    ______________________________________    Example 3             0.56 nm            4.5 × 10.sup.-8 A/cm.sup.2    Example 5             0.50 nm            9.5 × 10.sup.-9 A/cm.sup.2    ______________________________________

EXAMPLE 6

FIG. 18 is a sectional view of a film formed layer-by-layer on a Ptelectrode according to a further example of the invention. In FIG. 18,reference numeral 12 designates a SrTiO₃ film, which is relativelyeasily crystallized, and 13 designates a BaTiO₃ film, which isrelatively easily made amorphous. FIG. 19 shows the flow rate of sourcematerial solutions during formation of the SrTiO₃ film, which isdeposited for two minutes after starting the film formation (i.e.,during the period from the film formation starting time to 2 min) andfor another two minutes after the lapse of four minutes (i.e., duringthe period from four min to 6 min), and BaTiO₃ film, which is depositedfor two minutes after the lapse of the first two minutes (i.e., duringthe period from 2 min to 4 min), and for another two minutes after thelapse of the first six minutes (i.e., during the period from 6 min to 8min). Further, after forming the respective films, an annealing isapplied to them at 625° C. under a N₂ atmosphere for 10 sec. Theremaining conditions of film formation are same as those of Table 1.

Table 6 shows the electric characteristics of the CVD-BST film formed inthe layer-by layer film formation method compared with those of the filmformed in example 3, and it is to be understood from Table 6 thatforming the film layer-by-layer is more advantageous. Generally, it issaid that SrTiO₃ film is easier to crystallize than BaTiO₃ film. Thus,it is presumed that because a film of superior crystallization isobtained by forming a SrTiO₃ film at the initial stage of film formationand the BaTiO₃ film is relatively amorphous, the grain size is smallresulting in restraint of the leakage current. It is to be noted thatthe leakage current is large in prismatic crystal, but not in amorphouscrystal.

                  TABLE 6    ______________________________________           Film thickness t.sub.eq calculated                            Leakage current J.sub.L           in terms of oxide film                            (at +1.1 V)    ______________________________________    Example 3             0.56 nm            4.5 × 10.sup.-8 A/cm.sup.2    Example 6             0.52 nm            6.2 × 10.sup.-9 A/cm.sup.2    ______________________________________

EXAMPLE 7

FIG. 20 is a schematic diagram showing composition of the BST filmobtained according to an example of the high dielectric constant thinfilm forming method in accordance with the invention. In this example,for each DPM compound containing Ba, Sr or Ti, it is utilized thatthermal decomposition velocity on the substrate surface depends upontemperature only in the DPM compound containing Ti. A target compositionratio of the composite oxide at this time is Ba:Sr:Ti=1:1:2.

This composition ratio varies sensitively depending upon substratetemperature. For example, it is understood from FIG. 5 that when thesubstrate temperature is 500° C., the abundance ratio of Ti is increasedto the extent of Ba:Sr:Ti=1:1:3. It is understood that since thecomposition ratio depends largely upon the substrate temperature, thecomposition of a depositing film may be controlled by changing thesubstrate temperature during the process of film deposition. Formationof a region of large Ti composition ratio at a certain depth of a thinfilm of which composition ratio is Ba:Sr:Ti=1:1:2 is effective forimproving the insulating characteristic of the BST film. To achieve suchan improvement, it may be said effective to utilize the dependency ofthe composition ratio upon the substrate temperature in the mentionedfilm formation.

A result of an actual experiment is hereinafter described. A wafer washeated by infrared radiation, the flow rate of the solution sourcematerials was the same as that shown in FIG. 12, and the remainingconditions were same as those shown in Table 1. In this two step filmformation, a main layer was formed by deposition while heating asubstrate temperature to 500° C. by increasing the output of an infraredheater for 30 sec after the first two minutes (i.e., during the periodfrom first 2 min to 2 min and 30 sec) of six minutes, and then loweringthe substrate temperature to 420° C.

FIG. 20 shows a result of composition ratio in the film evaluated byAES. It is understood from FIG. 20 that, with regard to the portiondeposited at a substrate temperature of 420° C., the targetedcomposition ratio of Ba:Sr:Ti=1:1:2 was substantially achieved, and theabundance ratio of Ti was increased to about Ba:Sr:Ti=1:1:3 only at theportion where substrate temperature was raised.

The sharp variation shown in FIG. 20 means that this film formationmethod is effective for achieving a stepwise variation in thecomposition ratio distribution. It is to be noted that in supplying theDPM compounds containing Ba, Sr, and Ti, at least one compound issupplied to a reactor at a feed rate to be rate-determined by thermaldecomposition reaction temperature on the substrate surface and,therefore, it is possible to modulate a film composition easily andsharply by changing the substrate temperature during the process of filmformation. Thus, it became possible to deposit a composite oxide film ofthe desired component distribution. Such a sharp variation incomposition is never achieved merely by controlling the flow rate of thesource materials.

Table 7 shows the electric characteristics of the CVD-BST film actuallyobtained in comparison with example 3, and it is understood from Table 7that the leakage current is reduced. FIG. 21 shows a relation betweenthe composition ratio and the electric characteristics of the mainlayer, and it is understood from FIG. 21 that when controlling the flowrate of solution materials so that (Ba+Sr)/Ti ratio may be small, BaCO₃or Sr CO₃ is difficult to form, i.e., the amount of C contained asimpurity in the film is less, thereby decreasing the leakage current.When such a main layer is interposed, even if it is a thin film, a BSTfilm of less leakage current may be obtained.

Although variation in the substrate temperature is 50° C. in thisexample, a modulation in composition ratio enabling a sufficientcharacteristic change maybe achieved when the temperature change is 10°C. or more with reference to the relation shown in FIG. 5. Although DPMcompounds containing Ba, Sr, or Ti are employed as organo-metalliccompounds in this example, the same advantage may be achieved when twoor more organo-metallic compounds not affecting each other in thereaction of thermal decomposition are employed. For example, Ca, Pb, Bi,Ta, Nb, etc. are available as a metallic element to be contained in anorgano-metallic compound, other than Ba, Sr and Ti.

                  TABLE 7    ______________________________________           Film thickness t.sub.eq calculated                            Leakage current J.sub.L           in terms of oxide film                            (at +1.1 V)    ______________________________________    Example 3             0.56 nm            4.5 × 10.sup.-8 A/cm.sup.2    Example 7             0.54 nm            2.2 × 10.sup.-8 A/cm.sup.2    ______________________________________

EXAMPLE 8

FIG. 22 is a schematic view showing an example of the high dielectricconstant thin film forming method in accordance with the invention, bywhich the dependency on substrate surface is eliminated. In FIG. 22,reference numeral 14 designates a shower nozzle for feeding vaporizedraw materials together with N₂ gas, numeral 15 designates a heater, 16is a thermocouple for measuring heater temperature, 17 is a thermocouplefor measuring wafer surface temperature, and 18 is a conductive resin.FIG. 23 shows wafer surface temperatures on a substrate of variousmaterials measured by setting the thermocouple 17 shown in FIG. 22 onthe wafer surface.

The substrates whose temperatures are shown in FIG. 23 include, first, aPt(700 Å)/SiO₂ (5000 Å)/Si substrate composed of SiO₂ of 5000 Å inthickness formed on the entire surface of a silicon substrate of 0.6 mmin thickness, and a Pt film of 700 Å in thickness further formed on theentire surface of the silicon substrate. The Pt coating is 100%. Thesecond substrate, a Ru(2000 Å)/SiO₂ (5000 Å)/Si, is composed of SiO₂ of5000 Å in thickness formed on the entire surface of a silicon substrateof 0.6 mm in thickness, and a Ru film of 2000 Å in thickness furtherformed on the entire surface of the silicon substrate. The Ru coating is100%. The third substrate, SiO₂ (5000 Å)/Si, is composed of a SiO₂ filmof 5000 Å in thickness formed on the entire surface of a siliconsubstrate of 0.6 mm in thickness. The Ru coating and Pt coating are both0%. The fourth item shown is a Test Element Group (TEG), composed of aSiO₂ film of 5000 Å in thickness formed on the surface of a siliconsubstrate of 0.6 mm in thickness, and a Pt film of 700 Å in thicknessfurther formed thereon coating 35% of the surface.

The conductive resin 18 (see FIG. 22) is employed to exactly bring thethermocouple into contact with the wafer surface and measure exactly thewafer surface temperature. For example, when the heater set temperaturewas 510° C., the Ru upper surface temperature was 480° C., the Ptsurface temperature was 475° C., and the SiO₂ upper surface temperaturewas 430° C. Accordingly, it is to be understood that the wafer surfacetemperature varies depending upon the various materials coating thesilicon substrate. The following, "a" to "d," indicate the parametersinvolved in wafer surface temperature:

a: the kind of and the coating percentage of the material coating thesilicon substrate surface;

b: the thickness of the coating material;

c: the thickness of the silicon substrate; and

d: the heater temperature setting.

The wafer temperature varies according to variation of the parameters"a" to "d." When varying the wafer surface temperature, the compositionratio (Ba+Sr)/Ti of the metallic elements in the (Ba, Sr) TiO₃ film alsovaries. This is because, as shown in FIG. 5, when varying the wafersurface temperature, the deposition rate of Ba oxide and Sr oxide doesnot vary, but the deposition rate of Ti oxide varies greatly.

After all, the variation in the kind and coating percentage of thematerial coating the silicon substrate surface results in a variation inthe composition ratio (Ba+Sr)/Ti of the metallic elements in thedeposited film.

To cope with this, in case of variation in wafer surface temperature dueto variation in the parameters "a" to "d", it is preferable to controlthe flow rate of the source materials so that the variation indeposition rate may be free from (i.e., offset by) the variation inwafer surface temperature. As the deposition rate of Ti oxide variesdepending upon the wafer surface temperature and flow rate of Ti sourcematerial, the deposition rate may be kept constant by adjusting oneaccording to a variation of the other. Under the condition of keepingconstant the flow rate of Ba source material and Sr source material, itis preferable to use only the Ti source material flow rate among thethree source material flow rates as a parameter with which to affect themetallic element composition ratio (Ba+Sr)/Ti.

In this respect, the following ("a" to "e") are the parameters involvedin the metallic element composition ratio (Ba+Sr)/Ti of the depositedthin film when the flow rates of Ba source material and Sr sourcematerial are constant:

a: the kind and coating percentage of the material coating the substratesurface;

b: the thickness of the coating material;

c: the thickness of the substrate;

d: the heater temperature setting for heating the substrate; and

e: the flow rate of Ti source material.

In a first step according to the invention, a thin film which is asample film is deposited on a sample substrate by varying the parameters"a" to "e", and a composition ratio (Ba+Sr)/Ti of the metallic elementsis measured to accumulate data. In this step, the deposition of thinfilm is carried out establishing that flow rates of Ba source materialare constant. The accumulated data are preferably made into a table.

In the second step, before actually forming a BST film, a targetcomposition ratio (Ba+Sr)/Ti of the metallic elements is decided. Theparameters "a" to "c" are measured for the sample substrate on which thesample film was formed, and value of the parameter "d" is decided. Then,a flow rate of Ti source material (i.e., the "modified Ti sourcematerial flow rate") is obtained by calculating, on the basis of thetarget value of the composition ratio of the metallic elements and theparameters "a" to "d". This calculation may be performed easily byutilizing the table.

In the third step, the BST film of the target metallic composition ratio(i.e., a "target film") may be formed on the desired substrate (i.e., a"target substrate") by performing the film formation on the basis of theadjusted Ti source material flow rate obtained in this manner, and theflow rates of Ba source material and Sr source material established inthe first step. That is, the target film is formed on the targetsubstrate.

In the above method, even if there is a difference in the kind andcoating percentage of the material coating the silicon substratesurface, a BST film of the same metallic element composition ratio maybe formed. As a result, the dependency on the substrate surface may

FIG. 24 shows a relation between coating percentage of a wafer surfacecoated with Ru by patterning on SiO₂ at a heater set temperature of 510°C., for example, and temperature of the wafer surface. It is to beunderstood from FIG. 24 that the greater the coating percentage, thehigher the wafer surface temperature in proportion to the coatingpercentage. Accordingly, it is possible to vary the wafer surfacetemperature by varying the coating percentage on the basis of dataobtained by preliminary measurement (i.e., data obtained by measuringrepeatedly the data shown in FIG. 23 by changing the coating percentageand heater temperature setting). The deposition rate of Ti oxide due tothe variation in the wafer surface temperature may be obtained from FIG.25.

FIG. 25 is a diagram showing a relation between the wafer surfacetemperature and the deposition rate of Ti oxide, Ba oxide and Sr oxide.The deposition rate shown in the drawing may be associated with thecomposition ratio (Ba+Sr)/Ti of the metallic elements in the BST film.

There is a difference between FIG. 5 and FIG. 25 in the respect thatwhile FIG. 5 showing a deposition rate on a silicon substrate, FIG. 25shows a deposition rate on a wafer on the entire surface of which SiO₂of 5000 Å is formed and Ru film of 2000 Å is further formed thereon. Thesource material flow rate in FIG. 25 is the same as that shown in Table1.

It is to be understood from FIG. 25 that, even if the wafer surfacetemperature varies, the deposition rate of Ba oxide and Sr oxide arehardly affected thereby. That is, the deposition rate varies dependingupon the flow rate of Ba source material and Sr source material, but isnot affected by wafer surface temperature.

FIG. 26 is a diagram showing a relation between the Ti source materialflow rate, the wafer surface temperature, and the deposition rate ofmetal oxide. The flow rate of Ti source material necessary foreliminating the influence caused by a variation in wafer surfacetemperature on the composition ratio (Ba+Sr)/Ti of the metallic elementsis obtained from this diagram. Both flow rates of Ba source material andSr source material are same as those shown in Table 1.

For example, it is supposed herein that a target composition ratio(Ba+Sr)/Ti of the metallic elements is obtained on condition that Rucoating percentage is 100%, wafer surface temperature is 525° C. and Tisource material flow rate is 0.5 cc/min. Then, supposing that surfacetemperature of the wafer of which 10% is coated with Ru (the remaining90% is coated with SiO₂) is raised to 480° C., the deposition rate of Tioxide is reduced almost to a half on the mentioned condition that Tisource material flow rate is 0.5 cc/min, resulting in a large variationin the composition ratio (Ba+Sr)/Ti. To cope with this, the Ti sourcematerial flow rate is increased to 1 cc/min, whereby it becomes possibleto keep the deposition rate of Ti oxide constant and to keep also thecomposition ratio (Ba+Sr)/Ti constant.

In the foregoing process, although the variation in wafer surface due tovariation in the parameters "a" to "d" is described as first step, andthe elimination of influence due to the wafer surface temperature bychanging the flow rate of Ti source material is described as secondstep, it is also possible to understand this as one step in thefollowing manner. On condition that flow rates of Ba source material andSr source material are constant, parameters affecting the compositionratio (Ba+Sr)/Ti of the metallic elements of BST film are the mentioned"a" to "e".

Although the composition ratio (Ba+Sr)/Ti is affected, or influenced, bythe variation in the parameters "a" to "d", the influence due to thevariation in the parameters "a" to "d" may be eliminated by adjustingthe flow rate of Ti source material, which is the remaining parameter,"e," whereby the composition ratio (Ba+Sr)/Ti may be kept constant.

In the one-step approach mentioned above, the measurement of the wafersurface temperature may be omitted. It is similarly preferable to carryout the measurement of wafer surface temperature in order to control theflow rate of the Ti source material on the basis of the resultingtemperature measurement, as a matter of course.

To keep constant the composition ratio of the metallic elements of theBST film, in the thickness direction, the flow rates of Ba sourcematerial and Sr source material may be varied stepwise, while keepingthe flow rate of Ti source material constant, as shown FIGS. 10 and 12.

To keep constant the composition ratio of the metallic elements, in thethickness direction, and to keep constant the composition ratio(Ba+Sr)/Ti of the metallic elements of the BST film irrespective ofvariations in the kind and coating percentage of the material coatingthe wafer surface, the flow rate of Ti raw material, as shown in FIGS.10 and 12, may be shifted. The shift volume of the Ti raw material flowrate is a volume sufficient to eliminate the influence given to thecomposition ratio (Ba+Sr)/Ti due to the variation in kind and coatingpercentage of the material coating the wafer surface, and the shiftvolume may be obtained per FIG. 26.

Peak values in FIGS. 25 and 26 are obtained if Ba source material flowrate is constant at 0.04 cc/min and Sr source material flow rate isconstant at 0.03 cc/min, in the same manner as in Table 1.

It is also possible to obtain peak values by varying the constant flowrates of Ba source material and Sr source material. Here, theabove-mentioned steps 1 to 3 may be carried out on the basis of thevaried flow rates of Ba source material and Sr source material.

Because the deposition rate of Ba oxide and Sr oxide is in proportion tothe flow rate of the source materials irrespective of wafer surfacetemperature, the deposition rate of Ba oxide and Sr oxide in FIGS. 25and 26 shifts vertically in proportion to the flow rates of the Basource material and Sr source material.

As described above, by varying the flow rate of Ti source material, itis possible to obtain a BST film of almost the same film composition asa wafer coated entirely with Pt or Ru even if SiO₂ is exposed to thesubstrate due to the patterning of the Pt or the Ru. In this manner, thedependency on the substrate may be eliminated.

EXAMPLE 9

FIG. 27 is a schematic view showing an example of the high dielectricconstant thin film forming method in accordance with the invention. InFIG. 27, reference numeral 26 designates N₂ gas, numeral 27 designates aBa(DPM)₂ /THF mixed solution, 28 designates a Sr(DPM)₂ /THF mixedsolution, 29 is a TiO(DPM)₂ /THF mixed solution, 30 is a THF. 31 is avaporizer, 32 is a reaction chamber, 71 is a solution flow ratecontroller, 72 is a gas flow rate controller, 73 is a pressure valve, 20is a mixer, and 22 is a susceptor.

To vaporize the source material solutions smoothly by the vaporizer, itis necessary to spray them together with a carrier gas N₂. However, whenthe source material solutions touch N₂, the solvent is vaporized andvolatilized, and there is a possibility of the solidification of thesolid source materials, which could clog the pipe. To overcome thisproblem, in the invention there is no part where any source materialsolution touches N₂ directly and, therefore, it is possible to introducefirst the solvent THF into a line, then the source material solutions27, 28, and 29 one after another, whereby a gas-liquid mixture issmoothly and constantly supplied to the vaporizer without clogging. Withflow rates of the Ba, Sr, and Ti source material solutions set as shownin Table 1, it is estimated that amount of THF first introduced ispreferably 1/5 as much as the total flow rate of the Ba, Sr, and Tisource material solutions; in this sense, the solvent was introduced at0.2 cc/min. Pressure inside the vaporizer is more stable when a largeramount of N₂ is supplied, but the flow rate of the solution sourcematerial may vary when the amount of N₂ supplied is excessively large,and therefore amount of N₂ was set to 200 sccm. There is no particularintroduction order to be established among the Ba, Sr and Ti sourcematerial solutions.

The N₂ gas 26 is used just for pressurizing the source materialsolutions 27, 28, and 29 with respect to the THF 30, and drawing themfrom the container. The flow rates of the Ba(DPM)₂ /THF solution,Sr(DPM)₂ /THF solution, TiO(DPM)₂ /THF solution, and the THF solutionare controlled by the solution flow rate controller 71.

The thin film manufacturing apparatus in FIG. 27 may be used in othersituations, in which case the THF 30 is not always required.

EXAMPLE 10

FIG. 28 is a schematic view showing an example of the high dielectricconstant thin film forming apparatus in accordance with the invention.In FIG. 28, reference numeral 20 designates a mixer, numeral 21designates a substrate, 22 is a susceptor, 23 is a source material gasfeed pipe, 24 is a O₂ gas feed pipe, and 25 is an optical fiber formonitoring temperature. An infrared sensor 75 for detecting a surfacetemperature of the substrate 21 is provided on the end of the opticalfiber 25. A portion detected by the optical sensor is made of SiO₂ inevery wafer. Since the infrared sensor 75 is disposed in the O₂ pipe,even the DPM source material, which easily condenses on the lowtemperature portion, may stably be monitored without condensationforming on the end of the sensor.

Thus, it becomes possible to monitor the wafer surface temperature bythe infrared sensor 75 through the optical fiber, obtain an identicalBST film composition by varying the Ti source material flow rate, andeliminate the dependency on substrate, whatever material may be used inany part other than the detected part of wafer, no matter how thepatterning may be, and even if the surface area of SiO₂ occupied by Ptor Ru is different.

Control of the Ti source material flow rate may be achieved by theconventional manner. That is, the control is performed with the usualfeedback control, monitoring the wafer surface temperature by theinfrared sensor 75.

In case variation in the wafer surface temperature occurs due todeterioration or the like of the heater or susceptor, such a variationmay immediately be detected, and stability or consistency in the filmformation may be improved by changing or replacing the deterioratedheater or susceptor.

EXAMPLE 11

FIG. 29 is a schematic sectional view showing a capacitor structure of ahigh dielectric constant thin film as a further example in accordancewith the invention. In the drawing, reference numeral 63 designates anupper electrode (cell plate) made of Ru and deposited by a CVD methodhaving a superior coverage characteristic; 64 designates a highdielectric constant thin film, such as BST film, formed by a CVD methodfor example, 65 designates a lower electrode (storage node) made of Rudeposited, for example, by sputtering. A shoulder portion 66 is roundedwith a radius of curvature of 50 nm, and the side faces of the storagenode are not vertical (i.e., not 90°), but are inclined instead at 80°with respect to the substrate planar surface. Numeral 67 designates aninterlayer insulating film made of silicon dioxide (SiO₂), for example,and 68 designates a plug made of polysilicon film (PolySi) deposited bya CVD method having a superior coverage characteristic, for electricallyconnecting the storage node 35 and a gate transistor. A transistor isformed on the silicon substrate 1 between plugs 68.

A process for manufacturing the capacitor structure shown as an examplein accordance with the invention is now described with reference toFIGS. 30(a) to (e). The polysilicon (PolySi) is buried by a CVD methodhaving a superior coverage characteristic in a contact hole formed bylithography and etching on the interlayer insulating film 67 of silicondioxide. Then, the portion deposited on the surface of the interlayerinsulating film 67 is entirely removed by etching or chemical-mechanicalpolishing (CMP method), thus forming plug 68 (FIG. 30(a)).

Then, Ru film 65 of 2000 Å in thickness is deposited on plug 68 bysputtering, and is plasma-etched under an atmosphere of oxygen/chlorinemixed gas up to reaching the underlying interlayer insulating film 67using a 100 nm silicon oxide (SiO₂) film 71 as a mask FIG. 30(b)).Employed in this etching is magnetron RIE (reactive ion etching to whichmagnetic field is added), and of which etching conditions areestablished to be Cl₂ /(O₂ +Cl₂)=7.5%, 300 sccm in total gas amount, 30mTorr in reaction chamber pressure, 250 W in rf input, and 200 Gauss inmagnetic intensity. Under these conditions, the etching rate is 400Å/min, and when stopping the etching after overetching the underlyinginterlayer insulating film for 30 sec, the side face inclination of thestorage node becomes 80°. This inclination is controllable by varyingthe total gas amount.

Then, the shoulder portion 66 of the storage node is rounded by plasmasputter-etching under an atmosphere of inert gas, such as Ar, using thesame etching apparatus (FIG. 30(c)). The sputter-etching rate is 200Å/min when the Ar amount is 100 sccm, the reaction chamber pressure is 5mTorr, and the plasma input is 600 W, for example, and in case ofprocessing for 30 sec, the radius of curvature of the shoulder portion66 of the storage node is 50 nm, which is almost the same as the BSTfilm thickness. This radius of curvature is controllable by varying theprocessing time.

The entire surface of the storage node 35 of 200 nm in height, formed asmentioned above, is then coated with the BST film 64 of 30 nm inthickness by a CVD method (FIG. 30(d)), and the cell plate 33 is furtherformed by depositing Ru by a CVD method having a superior coveragecharacteristic (FIG. 30(e)).

The BST film 64 is prepared by the thin film forming method according tothe invention as mentioned in examples 1 to 9.

The interlayer insulating film 67 of silicon dioxide is provided with agroove. The side face of the storage node is inclined at 80°. Further,the high dielectric constant thin film BST 64 and the cell plate of Ru63 are deposited on the groove portion by a CVD method having a superiorcoverage characteristic, whereby it is possible to bury the cell plate63 more deeply than the lower end of the side face of the storage node.

In the capacitor formed as described above, when applying a voltagebetween the cell plate 63 and the storage node 65, since the radius ofcurvature of the shoulder portion of the storage node is 50 nm, which islarger than the BST film thickness of 30 nm, a field concentration isprevented as can be understood from FIG. 31 which shows a simulationresult of three-dimensional electric field strength. Consequently, theincrease in leakage current may be restrained as shown in FIG. 32. Thisis because, with the enlarged radius of curvature of the shoulderportion 66, the adhesive property of the BST film is improved.

The field concentration on the lower end of the cell plate may be alsoprevented by burying the cell plate 63 more deeply than the lower end ofthe side face of the storage node 65. Further, as a result of buryingthe cell plate 63 more deeply than the lower end of the side face of thestorage node 65, the entire side face of the storage node 65 may beeffectively utilized as a capacitor electrode, thereby increasing theelectric capacity. Furthermore, there is an advantage of solving theproblem that potentials of the storage nodes adjacent each otherinterfere or affect negatively with each other through the BST film inthe cut portion.

The groove of the interlayer insulating film is provided so as to formits side face with an inclination of 60°. Under an atmosphere of inertgas such as Ar and HF mixed gas, the groove of the interlayer insulatingfilm 67 of silicon dioxide is formed by plasma simultaneously with theremoval of the oxidation mask. At this time, it is possible to controlthe inclination of the side face of the groove in the interlayerinsulating film by optimizing the pressure in the reaction chamber,mixing ratio of Ar and HF, and plasma input. In this example, theinclination of the groove side face is 60°, which is smaller than the80° inclination of the storage node side face. The field concentrationon the lower end of the side face of the storage node is moderated byforming the inclination of side face of the groove of interlayerinsulating film 67° to be 60°, which is smaller than 80° inclination ofthe storage node side face.

EXAMPLE 12

FIG. 33 is a schematic sectional view of a capacitor structure shows asa further example of the high dielectric constant film in accordancewith the invention. In this example, the cell plate 63 is buried withthe same depth as the lower end of the side face of the storage nodeand, accordingly, field concentration is simultaneously moderated onboth the lower end 69 of the cell plate and on the lower end 70 of theside face of the storage node, thereby increase in leakage current beingrestrained. The BST film 64 may be prepared by the thin film formingmethod mentioned in examples 1 to 9.

Although BST is described as a material of high dielectric constantcapacitor in this example, the same advantage may be achieved by otherhigh dielectric constant material such as PZT as a matter of course.

What is claimed is:
 1. A high dielectric constant thin filmmanufacturing apparatus, comprising:a reaction chamber in which a thinfilm is formed by CVD on a substrate; a source material gas feed pipefor feeding a source material gas to said reaction chamber; an oxygengas feed pipe for feeding an oxygen gas to said reaction chamber; and aninfrared sensor disposed in said oxygen gas feed pipe for detecting asurface temperature of said substrate.